Friction characteristic of micro-arc oxidative Al2O3 coatings sliding against Si3N4 balls in various environments

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Friction characteristic of micro-arc oxidative Al2O3 coatings sliding againstSi3N4 balls in various environments

Fei Zhou a,⁎, Yuan Wang b, Hongyan Ding c, Meiling Wang d, Min Yu a, Zhendong Dai a

a Academy of Frontier Science, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, PR Chinab School of Communication, Machinery and Civil Engineering, Southwest Forestry University, Kunming, 650224, PR China

c Department of Mechanical Engineering, Huaiyin Institute of Technology, Huai’an 223001, PR Chinad School of Materials Science & Engineering, Nanjing University of Aeronautics & Astronautics, Nanjing, 210016, PR China

Received 26 November 2007; accepted in revised form 21 January 2008Available online 2 February 2008

Abstract

The alumina ceramic coatings were prepared on 2024Al alloy by micro-arc oxidation (MAO) technique. The phase structure of the MAOAl2O3 coating was determined using X-ray diffraction. The thickness and micro-hardness of the MAO Al2O3 coatings was measured using eddycurrent thickness equipment and micro-hardness tester. The friction property of MAO Al2O3 coatings sliding against Si3N4 ceramic balls wereinvestigated in air, water and oil by a ball-on-disk tribo-meter, and the worn surfaces of the MAO Al2O3 coatings were observed using scanningelectron microscope (SEM). The results showed that the MAO Al2O3 coatings mainly contained α-Al2O3 and γ-Al2O3 phase. The micro-hardnessof the polished MAO coatings was HV1740±87. With an increase in normal load and sliding speed, the friction coefficient in air increased from0.74 to 0.87, while decreased from 0.72 to 0.57 in water and 0.24 to 0.11 in oil. This indicates that the fluid lubrication could improve the frictionbehavior of the MAO Al2O3 coatings. The worn surfaces' observation indicated that the wear mechanism of the MAO Al2O3 coatings changedfrom abrasive wear in air to mix wear in water, and became microploughing wear in oil.© 2008 Elsevier B.V. All rights reserved.

Keywords: Aluminum alloy; Micro-arc oxidation; Friction; Lubrication

1. Introduction

Aluminum alloys possess high strength-to-weight ratio,good ductility, and are often used in automotive and aerospaceindustries. However, as far as the tribological machine parts areconcerned, the poor wear-resistance of aluminum alloys in airand water decreases the service life of machine components [1].Therefore, it is imperative to enhance their wear-resistance tofurther increase their service life in various environments.Ceramic coatings are well known to improve the wear resistanceof machine parts [2–6]. However, due to the elastic or plasticdeformation of aluminum alloy under mechanical loading, thethin hard coatings via PVD methods often exhibit limitedtribological performance [2–6]. Recently, the micro-arc oxida-tion (MAO) process technique can form a thick aluminum oxide

and other oxide ceramic coatings on aluminum alloys by plasmadischarging in an aqueous electrolyte solution on aluminumsurface under high voltage [7–13]. The MAO coatings haveexhibited better mechanical properties as compared with anodicoxide coatings and plasma spray ceramic coatings, and then canbe applied to many industrial fields, such as automotive, aero-space, medicine and textile engineering, etc [14].

Previously, many scientists have paid more attention tolooking for good MAO parameters [15–20] to obtain highquality MAO Al2O3 coatings on aluminum alloys. But for theMAO Al2O3 coatings' tribological properties, the friction andwear properties of the MAO coatings against different matingmaterials have already been investigated in air [9,10,14,16,21–24]. In fact, lots of machine parts made of aluminum alloys areperformed in various environments such as water and oil. Underlubrication condition, the presence of sub-micrometer, surface-connected porosity in the MAO coatings is beneficial to ob-taining low friction and good wear resistance [25]. However, the

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tribological properties of MAO coatings in water and oil havenot yet studied in detail. Therefore, the purpose of this paper isto investigate the friction and wear characteristic of MAOAl2O3

coatings in air, water and oil and discuss the influence ofambient environment on the wear mechanism of MAO Al2O3

coatings.

2. Experimental procedure

2.1. Materials and specimens

The extruded aluminum alloy 2024Al was used as a sub-strate. The chemical composition and mechanical properties of2024Al were shown in Tables 1–2. The disk specimens(ø30 mm×4 mm) were gotten via machining 2024Al alloybars. The uniform thickness of all specimens was ensured bygrinding less than 0.1 mm of tolerance from both sides of thedisk surface. One side of disc was polished to remove thegrinding damage and any surface irregularities. The surfaceroughness of the polished surface was 0.1 μm.

2.2. MAO coatings deposition

Prior to MAO coating deposition, the disk samples weredegreased in a water solution with 10 vol.% H2SO4 at 60–80 °Cfor 5 min, and washed with distilled water. An aqueous solutionof electrolyte was prepared with chemically pure 10 g/LNa2PO3, 8 g/L Na2SiO3 and some additives. Alumina ceramiccoatings were synthesized using a MAO unit which consists ofan insulted electrolyte bath and a high voltage power supplygenerating saw tooth pulses with the maximum voltage ampli-tude of about 1.5 KV and maximum frequency of 3 KHz. Oneoutput of the power supply was connected with a stainless steelbath, and another with a specimen was immersed into elec-trolyte. The MAO Al2O3 coatings were deposited on 2024 Alfor 30 min. A constant current density 5 A/dm2 on specimen'ssurface was adjusted by voltage control during processing.The electrolyte temperature was kept below 50 °C. After theMAO Al2O3 coatings deposition, the samples were ground andpolished to the surface roughness of 0.6–0.8 μm (Ra) using SiCground paper, and then ultrasonically cleaned in acetone for20 min.

2.3. Microstructure, thickness and microhardness of Al2O3

coatings

The MAO Al2O3 coatings' topography was observed byusing QUANTA 2000 scanning electron microscopy (SEM)(FEI, USA) and their structure were determined by D8-AdvanceX-ray diffraction (XRD) (Bruker, Germany). The coatings'thickness was measured by using MINITEST1100 eddy currentthickness gauge. The coatings' micro-hardness was measured at4.9 N for 10 s by a digital micro hardness tester (HXD-1000TM,China). The micro-hardness measurement was repeated forthree times, and the micro-hardness value in here was the meanvalue of micro-hardness for three times.

2.4. Friction tests

Prior to wear test, Si3N4 balls with diameter of 4 mm wereultrasonically cleaned in acetone for 15 min and then dried inair. The wear experiments were performed using ball-on-flattribo-meter (UMT-2, CETR, USA, Fig. 1) at room temperaturein air, distilled water and sewing machine oil. The normal loadwas in the range of 5–10 N, and the sliding speed varied in therange of 0.05–0.15 m/s. The distilled water and oil was addedonto rubbing surfaces using an injection-tube. The frictioncoefficients were obtained directly from the above-mentionedtribo-meter's computer. After testing, the three-dimensional andsectional morphologies of wear tracks on the MAO Al2O3

coatings were determined using a MicroXAM™ non-contactoptical profilometer (ADE Phase-Shift, USA), and the weartrack of MAO Al2O3 coatings were observed by SEM (FEI,USA).

3. Results and discussion

3.1. Microstructure, thickness and micro-hardness of MAOAl2O3 coatings

Fig. 2 shows the microstructure and XRD spectrum for theMAO Al2O3 coatings. As seen in Fig. 2(a), the coatings' surfaceexhibited crater-mouth like traces and was covered with manyplasma discharge products. When a plasma discharge appeared,

Table 1Chemical composition of 2024 aluminum alloy

Element Si Fe Cu Mn Mg Al

Content(wt.%) b0.50 b0.50 3.8–3.9 0.3–0.9 1.2–1.8 Balance

Table 2Mechanical properties of 2024 aluminum alloy

Materials Yield strength Maximum strength Elongation Microhardness

σs (MPa) σb (MPa) δ(%) HV0.01

2024 Al 267.9 545.5 17 170Fig. 1. Schematic diagram of ball-on-disk for UMT-CETR tribo-meter.

3809F. Zhou et al. / Surface & Coatings Technology 202 (2008) 3808–3814


plasma, thermal chemical and electrochemical reactions oc-curred between aluminum and oxygen and other ions from theelectrolyte. Although these reactions were quite complex innature, but finally the high melting point products (alumina)should be formed in the discharge channels and ejected into thecoating surface along the channel wall and then deposited in thechannel and on the surface at high discharge temperature andpressures. When the plasma discharge extinguished, it left acrater-mouth like product on coating surface. It is evident fromFig. 2(b) that the MAO coating consisted mainly of α-Al2O3

and γ-Al2O3. The initial thickness and the micro-hardness ofMAO Al2O3 coatings were 35 μm and HV1211±60, whilethose of polished MAO Al2O3 coatings were 25 μm andHV1740±87. In fact, the α-Al2O3 phase was thermodynami-cally stable at all temperature, whereas γ-Al2O3 phase wasmetastable. Refs.[10,16,21] have reported that the contents ratioof α-Al2O3/γ-Al2O3 phases increased with an increase in thecoatings' thickness from surface to interface due to the coolingrate difference between outlayer region and internal region. Thismeans that the coating was composed of α-Al2O3 compact layerand γ-Al2O3 loose layer at the pores. Because the mechanicalproperty of α-Al2O3 phase was better than that of γ-Al2O3

phase, so the hardness of internal region was higher than that ofoutlayer region.

3.2. Friction behavior of MAO Al2O3 coatings sliding againstSi3N4 balls in various environments

3.2.1. Influence of normal loadThe friction behavior of the MAO Al2O3 coatings sliding

against Si3N4 ball in air, water and oil at various normal loads isillustrated in Fig. 3. It is evident that the running-in period of theMAO Al2O3/Si3N4 tribopair in air was longer than that in waterand oil at 0.05 m/s. As the normal load increased, the running-inperiod became short. In air, with an increase in the slidingdistance, the friction coefficient at 5 N increased from 0.68 to0.77, while at 7.5 N or 10 N, the stable friction coefficientfluctuated in the range of 0.72–0.77 or 0.80–0.84, respectively.If the distilled water was added onto friction surface, the frictioncoefficient at 5 N increased rapidly from 0.45 to 0.65 as thesliding distance was shorter than 15 m, and then rose from 0.65to 0.70 as the sliding distance increased from 15 to 84 m.Finally, it varied in the range of 0.70–0.72. When the normalload was 7.5 N or 10 N, the friction coefficient increasedsuddenly from 0.31 to 0.64 or from 0.23 to 0.60 at the slidingdistance shorter than 45 m, respectively. Then the frictioncoefficient fluctuated in the range of 0.64–0.71 or 0.60–0.68with further sliding. When oil was induced onto rubbingsurface, the friction coefficient at 5 N increased from 0.18 to0.24 as the sliding distance lower than 15 m, and then fluctuatedin the range of 0.24–0.25 with sliding distance. If the normalload was 7.5 N, the friction coefficient initially increased from0.16 to 0.18 within the shortest sliding distance of 8 m, and thenkept a constant value of 0.18 with further sliding. But at 10 N,the friction coefficient first increased from 0.13 to 0.16 assliding distance was shorter than 45 m. As the sliding distanceincreased, it fluctuated slightly in the range of 0.16–0.17. Fig. 4shows the variation of mean stable friction coefficient (μm) withnormal loads in various environments. With an increase innormal load, the mean stable friction coefficient (μm) in airincreased, while in oil it decreased. When the sliding speed waslower or higher than 0.1 m/s, the friction coefficient in waterdecreased linearly with normal load. But at 0.1 m/s, the frictioncoefficient first decreased from 0.69 to 0.63 at 7.5 N, then

Fig. 2. Microstructure (a) and XRD spectrum(b) for the MAO Al2O3 coatings.

Fig. 3. Friction behavior of MAO Al2O3 coatings sliding against Si3N4 ball atvarious normal loads in air, water and oil.

3810 F. Zhou et al. / Surface & Coatings Technology 202 (2008) 3808–3814


increased to 0.67 at 10 N. The mean stable friction coefficient(μm) of the MAO Al2O3/Si3N4 tribopair in air varied in therange of 0.74–0.87, but it varied in the range of 0.57–0.72 inwater and 0.11–0.24 in oil. This indicates that the highestfriction coefficient was obtained in air, while the lowest frictioncoefficient was obtained in oil.

3.2.2. Influence of sliding velocityFig. 5 shows the friction behavior of the MAO Al2O3/Si3N4

balls tribo-pairs at various sliding speeds in three kinds ofenvironments. In air, the running-in period of the MAO Al2O3/Si3N4 tribo-pairs at 0.05 m/s or 0.15 m/s was 50 m, whilebecame 125 m at 0.1 m/s. With an increase in sliding distance,the friction coefficient first increased from 0.23 to 0.80 at0.05 m/s, to 0.82 at 0.1 and 0.15 m/s, and then fluctuated in therange of 0.80–0.82, 0.82–0.85 and 0.83–0.88, respectively.When the distilled water was added onto the friction surface,at the sliding speed lower than 0.1 m/s, the running-in periodwas 125 m and the friction coefficient first increased from 0.24to 0.66, then approached to 0.68 with further sliding. But at0.15 m/s, the running-in period was 75 m, and the friction

coefficient first gradually increased from 0.24 to 0.50, thenvaried in the range of 0.50–0.60 with an increase in slidingdistance. If the friction surface was covered with oil, the runningperiod was 10 m. With an increase in sliding distance, thefriction coefficient fluctuated in the range of 0.16–0.17 at0.05 m/s, 0.14–0.15 at 0.1 m/s and kept a constant value of 0.11at 0.15 m/s. The variation of mean stable friction coefficient(μm) with sliding speeds as the MAO coatings slid against Si3N4

ball in air, water and oil is displayed in Fig. 6. The mean stablefriction coefficient (μm) of the MAO Al2O3/Si3N4 tribo-pairs inair increased linearly with sliding speed at various normal loads,while in water or oil, it decreased linearly with sliding speed at anormal load lower or higher than 7.5 N. When the normal loadwas 10 N and the sliding speed increased, the friction coefficientin water or oil increased to the highest value at 0.1 m/s, and thendecreased. The results in Figs. 4 and 6 indicates that, withincreases in the normal load and the sliding speed, the frictioncoefficient in air increased while decreased in water and oil.The MAO Al2O3/Si3N4 tribopair displayed the lowest frictioncoefficient in oil.

3.3. Observation of worn surface for MAO Al2O3 coatings invarious environments

Fig. 7 shows the three-dimensional and sectional morphol-ogies of wear track on the MAO Al2O3 coatings sliding againstSi3N4 balls at 10 N and 0.1 m/s in various ambient en-vironments. In air, the rough worn surface was observed and thewear track became wider and deeper. If the lubricant such aswater or oil was added onto friction surfaces, the worn surfacebecame shallow and narrow. In water, the wear track of theMAO Al2O3 coatings became smooth and flat, while in oil, therougher wear track surface was observed. The results fromFigs. 7(b), (d) and (f) indicate that the wear resistance of theMAO coatings in air was lowest while highest in oil. To knowthe wear track surfaces' differences in various environments, thewear track surfaces were observed by SEM. As seen in Fig. 8, inair, the wear track topography at 5 N was covered with manyflakes and original discharge channels (Fig. 8(a)), while at 7 N,the worn surface became flat and compacted (Fig. 8(b)). But at

Fig. 5. Friction behavior of MAO Al2O3 coatings sliding against Si3N4 ball atvarious sliding speeds in air, water and oil.

Fig. 6. Variation of the mean stable friction coefficient with sliding speeds as theMAO coatings sliding against Si3N4 ball in air, water and oil.

Fig. 4. Variation of the mean stable friction coefficient with normal loads as theMAO Al2O3 coatings sliding against Si3N4 ball in air, water and oil.



10 N, the MAO Al2O3 coatings with larger area were flakedaway and the fresh surface with many discharge channels wasobserved on the worn surface (Fig. 8(c)). If the distilled waterwas added onto the contact surface, the worn surface becamesmooth and flat besides original discharge channels at thenormal load lower than 7.5 N (Figs. 8(d) and (e)). But at 10 N,

aside from original discharge channels, the smooth and compactarea was enlarged (Fig. 8(f)). When the lubricant became oil, theworn surface exhibited discharge channels (Figs. 8(g)–(i)),similar to the original MAO coatings' surface. This indicatedthat the wear mechanism of the MAO Al2O3 coatings in threekinds of environments were different from each other.

Fig. 7. Three-dimensional and sectional morphologies of wear tracks on the MAO Al2O3 coatings as sliding against Si3N4 balls at 10 N and 0.1 m/s in air (a, b), water(c, d) and oil (e, f).



3.4. Discussion

The results in Figs. 3–7 showed that the ambient conditionshad an important influence on the friction property of the MAOAl2O3 coatings. For the MAO Al2O3/Si3N4 tribo-pairs, thefriction coefficient was highest in air, while lowest in oil.Generally, the friction and wear behavior at contact surface waslargely governed by the physical condition of the contactinginterface and the chemical interactions between the slidinginterfaces and environment. As seen in Figs. 4 and 6, as thenormal load and sliding speed increased, the friction coefficientincreased in air while decreased in water and oil. Tian [10]indicated that γ-Al2O3 layer has poor antiwear ability comparedwith α-Al2O3 internal layer. As the MAO Al2O3 coatings slidagainst Si3N4 balls in air, the MAO Al2O3 coatings were peeledoff to form the wear debris via ploughing the outer layer andmicrofracturing the internal hard coatings. The wear debris wasground to form fine powder left on the worn surface. After longtime, the fine powder was compacted or removed, and then theflat or rough worn surfaces on the MAO coatings were obtained.Thus, the dominant wear mechanism of MAO Al2O3 coatingswas abrasive wear. Furthermore, the oxidative reaction of Si3N4

to SiO2 easily occurred in air at the contact surface. Althoughthe flowing phase of oxidative debris was expected to providevery low friction, a too low viscosity reduced load carryingcapacity and gave rise to the highest friction values in air [26].

But in water, the wear volume of the MAO coatings was lowerthan that in air, and the wear surfaces became smooth and flat(Fig. 7(b) and Fig. 8(d)–(f)). Refs. [27–31] indicated that thetribochemical wear was characterized by a smooth and flatsurface and the delamination and dissolution of hydrationreaction products. It is evident that the tribochemical reactionsuch as hydration reaction between tribo-materials and waterduring sliding friction period easily occurred [27]. This in-dicates that the wear debris reacted with water to form lubri-cation gels such as Al(OH)3 or Si(OH)4 at the contact surface.Thus, the wear mechanism was mix wear of mechanical wearand tribochemicalwear. According to previous study on ceramics'water lubrication [27–31], the MAO Al2O3/Si3N4 tribo-pairsshould display low friction behavior. But her, higher frictioncoefficient in the range of 0.57–0.72 was obtained. This indicatedthat the discharge channels damaged water lubrication film, andthen the load carrying capacity for MAO Al2O3/Si3N4 tribo-pairsreduced, so the higher friction values in water were obtained. Ifthe oil lubricant was added onto friction surface, the frictioncoefficient and wear volume of MAO Al2O3 coatings exhibitedthe lowest values among three kinds of environments. When thenormal load and sliding speed increased, the friction coefficientdecreased. Generally, theMAO coatings are assumed to be almostfully dense. However, J.A.Curran and T.W. Clyne [25] reportedthe presence of sub-micrometer, surface-connected porosity atlevels of the order of 20% in theMAOcoatings. This porositymay

Fig. 8. SEM photographs of wear tracks on MAO Al2O3 coatings as sliding against Si3N4 balls at 0.15 m/s in various environments.



at least partially account for the observed high capacity for liquidimpregnation. Furthermore, the discharge channels on the MAOcoatings could serve either as a micro-hydrodynamic bearing, amicro-reservoir for lubricant, or a micro-trap for wear debris [32]during sliding friction tests. The oil lubricative film with higherload carrying capacity was formed at the contact zone, and thewear debris was formed via ploughing the outer layer. Thus, lowfriction and low wear for the MAO Al2O3/Si3N4 tribopair in oilwere obtained simultaneously, and the wear mechanism of MAOAl2O3 coatings was microploughing wear.

4. Conclusions

The friction and wear behaviors of the MAO Al2O3 coatingsagainst Si3N4 balls were investigated in air, water and oil atvarious normal loads and sliding speeds. The conclusions aresummarized as:

(1) The MAO Al2O3 coatings on 2024 Al alloy mainly con-tainedα-Al2O3 and γ-Al2O3 phase, and the micro-hardnessof the polished MAO Al2O3 coating was HV1740±87.

(2) With increases in normal load and sliding speed, thefriction coefficient of the MAO Al2O3/Si3N4 tribopair inair increased from 0.74 to 0.87, while in water and oil, thefriction coefficient decreased from 0.72 to 0.57 and 0.24to 0.11.

(3) Thewearmechanism of theMAOAl2O3 coatings changedfrom abrasive wear in air to mix wear in water, and finallybecame microploughing wear in oil.

Acknowledgements

This work was supported by the Senior Talent Program ofNanjing University of Aeronautics and Astronautics (NUAA)(2006) and the National Natural Science Foundation of Chi-na (NNSFC) (No.50675102 and No.50635030) as well asthe Natural Science Foundation of Jiangsu Province (NSFJS)(No.BK2007529).Wewould like to acknowledgeNUAA,NNSFCand NSFJS for financial support.

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Friction characteristic of micro-arc oxidative Al2O3 coatings sliding against Si3N4 balls in various environments

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